Over the weekend, many investors had been expecting Treasury bond yields to rise initially because of the rating downgrade, and planned to buy as rates went up, Comiskey said.

But as is often the case in markets, the crowd doesn’t get what it wants.

It also helped the Treasury market, of course, that stock prices collapsed worldwide Monday, putting a huge amount of cash in equity sellers’ hands. They had to invest it somewhere fast.

The Dow Jones industrial average closed down 634.76 points, or 5.6%, at 10,809.85, the lowest since October.

Buyers also flocked to German and Canadian government bonds, among others, as foreign stock markets crumbled. The German DAX stock index sank 5%. Canada’s main share index slid 4%.

Next up for the Treasury market: The Federal Reserve’s meeting Tuesday. The betting is that the Fed isn’t yet ready to announce a new bond-buying program to pump money into the financial system, but that could change if stock markets are in another freefall Tuesday.

The Treasury also will be selling $72 billion in new debt over the next three days: $32 billion in three-year notes Tuesday, $24 billion in 10-year notes Wednesday and $16 billion in 30-year bonds Thursday.

Those auctions will show whether foreign investors, in particular, still want Treasuries at these low yields.